In situ?7Li, Li/Ca, and Mg/Ca analyses of synthetic aragonites Academic Article uri icon

abstract

  • In situ secondary ion mass spectrometry (SIMS) analyses of ?7Li, Li/Ca, and Mg/Ca were performed on five synthetic aragonite samples precipitated from seawater at 25°C at different rates. The compositions of ?7Li in bulk aragonites and experimental fluids were measured by multicollector inductively coupled plasma–mass spectrometry (MC-ICP-MS). Both techniques yielded similar ?7Li in aragonite when SIMS analyses were corrected to calcium carbonate reference materials. Fractionation factors ?7Li/6Li range from 0.9895 to 0.9923, which translates to a fractionation between aragonite and fluid from ?10.5‰ to ?7.7‰. The within-sample ?7Li range determined by SIMS is up to 27‰, exceeding the difference between bulk ?7Li analyses of different aragonite precipitates. Moreover, the centers of aragonite hemispherical bundles (spherulites) are enriched in Li/Ca and Mg/Ca relative to spherulite fibers by up to factors of 2 and 8, respectively. The Li/Ca and Mg/Ca ratios of spherulite fibers increase with aragonite precipitation rate. These results suggest that precipitation rate is a potentially important consideration when using Li isotopes and elemental ratios in natural carbonates as a proxy for seawater composition and temperature.

publication date

  • March 2011